1. Field of the Invention
This invention generally relates to a communication network, and, in particular, to such a communication network in which a plurality of terminals, located remotely from one another, are connected through nodes and transmission lines and the connecting conditions among the transmission lines at the nodes are controlled to carry out a desired communication between arbitrarily selected source and destination terminals.
2. Description of the Prior Art
Heretofore, several proposals have been made as to a communication network which is formed by connecting a plurality of terminals, such as general purpose computers, exclusive purpose computers, memory devices, terminal controllers and printers, which are remotely spaced apart from one another through nodes and transmission lines, whereby communication may be carried out between arbitrarily selected source and destination terminals. For example, the coaxial bus structure as disclosed in a literature by R. M. Metcalfe and David R. Boggs, entitled "Ethernet: Distributed packet switching for local networks", CACM, Vol. 19, No. 7, July 1976, 395-404 pages, and the loop structure as disclosed in a literature by D. J. Farber, entitled "A Ring Network", Datamation, Vol. 21, No. 2, February 1975, 44-46 pages, are well known. Theses coaxial bus and loop structures are shown in FIGS. 1 and 2, respectively.
In the coaxial bus structure shown in FIG. 1, a plurality of terminals 3a-3e are connected to corresponding coaxial cables 1a, 1b and 1c via taps or T-connectors 2. As shown, provided as interposed between any one of the terminals 3a-3e and the corresponding tap 2 is an appropriate device 4, such as an input/output buffer, an input/output interface and transceiver, which is determined in accordance with the kind of the associated terminal. A terminator 5 is provided at each end of any of the coaxial cables 1a-1c, and the terminator 5 carries out impedance matching at the cable end so as to prevent the reflection of a signal from occurring. A coupling between two different coaxial cables is made via a repeater 6. In such a prior art coaxial bus structure, the network may be easily expanded by simply providing additional repeaters 6 and coaxial cables 1, and one or more of local terminals 3 may be easily removed without adversely affecting the overall performance of the network. However, since this network is, in essence, constructed by interconnecting separate coaxial cables 1 using repeaters 6, its reliability in operation as well as speed of operation cannot be expected to be very high.
For example, in the structure shown in FIG. 1, when the repeater 6b malfunctions, it is impossible to carry out communication between the terminal 3c and either one of the terminals 3d and 3e. In order to avoid such a situation, an additional repeater may be provided; however, this then causes the entire system to become larger in size and more complex in structure, and thus this approach is not advantageous. Moreover, since communication between those terminals which are connected to different coaxial cables, e.g., between the terminals 3a and 3c, or 3b and 3d, is necessarily carried out through the associated repeater 6a and the function of the repeater 6 is to match signals between the coaxial cables connected thereto, the communication speed depends upon the performance of the repeater. In addition, it is true that the communication speed of the coaxial bus structure shown in FIG. 1 may be increased by using optical fibers as transmission lines; however, since high impedance fiber optic T-connectors are not available at present, technological difficulties will be encountered if an attempt is made to use optical fibers in the Ethernet structure of FIG. 1.
On the other hand, in the loop structure shown in FIG. 2, a plurality of nodes 8 and a supervisor 9 are provided as connected to a loop-shaped transmission line 7, and individual terminals 3 are connected to the corresponding nodes 8 or the supervisor 9. In such a loop structure, use may be made of an optical fiber to form the transmission line 7 in order to increase the communication speed; however, the fundamental drawback of the loop structure is a lack of reliability. Stated in detail, in the loop structure as shown in FIG. 2, because of the fact that a plurality of nodes 8 and the supervisor 9 are serially connected, failure of only one of the nodes and/or a local failure in the transmission line will result in the total system failure. Besides, an expansion of the system is rather limited partly because of the nature of the loop structure, which is essentially a closed system, and partly because of the existence of the supervisor 9, which functions as a central controller of the system.